There is growing demand for higher rates of transmission in an access network. Because of this growing demand coupled with higher competition among network operators in the broadband market and price erosion on optical components, an increased interest is seen in fiber-based access technologies. Currently, there are several operators considering the deployment of various types of fiber access network systems.
FIGS. 1A-1C illustrate existing fiber optics networks. The various configurations depicted in FIG. 1 include a central office 10 with an optical line termination (OLT) 12 having a link to an optical network unit (ONU) 14 and/or optical network termination (ONT) 24 leading to a home network 16. FIG. 1A illustrates a first step in replacing copper lines with fiber lines by placing the ONU 14 in the vicinity of a user located in the home network 16. A point-to-multipoint fiber network 18, referred to as an optical distribution network (ODN) connects the OLT to the ONU/ONT to provide a fiber-to-the-cabinet (FTTcab) link in relatively close proximity to the home network. A copper link 20 between the ONU and a network termination (NT) 22 is thereby shortened, allowing higher-rate DSL transmission modes, such as VDSL2, instead of rate-limited ADSL2+, technology that is today widely deployed to span long distances between the central office and the user premises.
FIG. 1B illustrates a configuration in which the fiber link is terminated closer to the home network (for example, building/business or curb) utilizing a very short copper-length (house wiring) bridged by DSL or Ethernet (for example, fiber-to-the-building/business (FTTB) and fiber-to-the-curb (FTTC)). The rationale behind this kind of architecture is that fiber deployment becomes more expensive the closer it extends toward the user. Thus, fiber is provided to a specific point prior to the customer premise.
FIG. 1C illustrates a configuration in which the copper link is totally replaced by the fiber link from the central office to the user (i.e., fiber-to-the-home (FTTH)). FIG. 1C illustrates a migration to all-optical access with the fiber link 18 extending from the OLT 12 to an optical network termination (ONT) 24 at the customer premises.
Optical networks can be split into two families of networks: active and passive optical networks. If the optical distribution network (ODN) 18 contains active equipment, it is considered an active network. If the ODN is totally passive, the system is called a passive optical network (PON) and mainly exists in a point-to-multipoint (p2 mp) architecture. Point-to-point (p2p) structures are typically available (fiber-based Ethernet) for active optical networks.
PONs have gained great attention in the last few years due to the low cost of utilizing the p2 mp scheme (component sharing), which utilizes a fiber-frugal tree-topology, low maintenance (no remote powering in the FTTH configuration), and failsafe performance advantages (e.g., high meantime between failure, no active parts).
FIG. 2 illustrates a typical architecture of an existing PON 50. The PON includes a user network interface (UNI) 52 at a T reference point, an adaptation function (AF) 54, an ONU 56 or ONT 58, wavelength division multiplex (WDM) modules 60 and 62, network elements 64 and 66, an optical splitter 68, a service node interface (SNI) 70 at a V reference point, and a service node function 72. The letter S denotes an optical connection point/splice on the optical fiber just after the OLT 74 for downstream traffic and just after the ONU 56 for upstream traffic. The letter R denotes an optical connection point/splice on the optical fiber link just before the ONU for downstream traffic and just before the OLT for upstream traffic. An ODN 80 is also shown between connection points R and S.
The OLT 74 sends data downstream to all ONUs 56 and/or ONTs 58 via the ODN 80 using time division multiplexing (TDM). In the upstream direction, different ONUs 56 and/or ONTs 58 may be granted timeslots to communicate data to the OLT via a time division multiple access (TDMA) scheme controlled by the OLT. Upstream and downstream data transfer is separated on different wavelengths. In addition, video-overlay on a separate wavelength (e.g., wavelength division multiplying WDM) is typically supported by most PONS. The ODN consists of a common trunk fiber, a passive power splitter forking up to different users, and user-individual drop-fibers. The splitter is commonly placed in the field at a remote node (RN, not shown in FIG. 2).
There are currently three implementations of PON schemes. The implementations are Ethernet PON (EPON), Broadband PON (BPON), and Gigabit PON (GPON). Table 1 below illustrates the major PON technologies and properties of the technologies.
TABLE 1CharacteristicsEPONBPONGPONStandardIEEE 802.3ahITU-T G.983ITU-T G.984ProtocolEthernetATMATM/EthernetRates (Mbps)1244 up/1244 down622/1244 down1244/2488 down155/622 up155 to 2488 upSpan (km)102020No. of splits163264
As seen in Table 1, GPON, as a successor to BPON, is the most advanced system in terms of protocol rates, total span (trunk plus drop span), and the number of users per OLT (split-ratio).
FIG. 3 illustrates an existing GPON system 100 configuration. The GPON system includes an ONU 102 with a UNI 104, an ONU 106 with a UNI 107, a splitter 108, and an OLT 110 with a SNI 112. For the ranging procedure to work, the maximum logical differential distance between farthest and nearest ONU must be less than 20 kilometers. The total maximum reach is specified to 60 km and is limited by timing constrains on the service layer. For standard Class B+ optics, a physical reach of 20 km on a 32 split are practical. On full split and maximum rate, the total physical reach is specified as 10 kilometers by keeping the same trunk-drop ratio limitation.
There are available systems providing various GPON OLT system functions. Such systems may include switch fabric and GPON optics as discussed above. In the upstream direction, a burst clock and data recovery (CDR) unit may handle the TDMA data from the different ONUs and ONTs. In both directions, serializer-deserializer (SERDES) units provide serial/parallel conversion between physical media dependent and the GPON Transmission Convergence (GTC) stack specified.
Although there are PON systems having a rate-reach product in the range of 10 Gbps·km (EPON, BPON) to 50 Gbps·km (GPON) and maximum split-ratios of 16 to 64, there is an increasing demand for higher numbers, both in rate (towards 10 Gbps, 40 Gbps to enable 1 Gbps per user) and reach (60 km physical for short backhaul, and larger 100 km for long backhaul). As currently under discussion in IEEE 802.3av (10 Gbit/s Ethernet Passive Optical Network task force) and ITU-T Full service Access Node (FSAN) next generation access (NGA), the next generation of PONs must extend rates at least 10 Gbps downstream and 2.5 Gbps upstream.
There are currently no solutions providing all of the performance enhancements desired. Regarding the desired increase in rate, there are several problems to overcome when increasing a GPON to a 10GPON system. In particular, 10GPON systems must co-exist with the regular GPON on the same fiber (fiber-lean scenario). Moreover, more powerful optical components are necessary.
Regarding the desired increase in reach, current PON systems (as well as p2p Ethernet-based systems) are limited in the total span and, thus, are not appropriate for medium or long backhaul scenarios which virtually shift the central office up to the transport domain to extend the passive ODN with all its advantages. In that sense, central offices can be consolidated, reducing operational expenses.
Systems with trunk-spans greater than 100 km will be necessary in the future without any major modifications on the GPON system.